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The Colorado River

The Colorado River

The Colorado River is by far the largest source of surface water for a thirsty land. About five million years ago its silt-filled waters carved the Grand Canyon and formed the spectacular Gore, Glenwood, De Beque, Glen, and Cataract Canyons. Settlements on the Colorado Plateau have relied on the river for approximately 12,000 years. Today it is the main water source for 25 million people. Formed in Colorado’s Rocky Mountain National Park, the Colorado snakes 1,450 miles through five states and is tamed by over seven dams. Every river in Arizona drains into the Colorado or one of its tributaries. This mighty river used to flow into the Gulf of California after reaching its delta in Mexico—a delta that supported hundreds of animal and plant species. The Colorado River has reached its delta only five times since 1983.

Arizona gets about 40% of our water directly from the Colorado River. But we’re not the only ones. Seven states and Mexico all depend on the Colorado River—not to mention a vast array of wildlife. The Colorado is divided into two major management areas: the Upper Colorado Basin and the Lower Colorado Basin. Wyoming, Utah, Colorado, and New Mexico receive water from the Upper Colorado. Nevada, California, and Arizona depend on the Lower Colorado.

These seven states have rights to 15 million acre-feet of water each year, with an additional 1.5 million acre-feet allocated to Mexico. (Arizona receives 2.8 million acre-feet annually.) However, the Colorado River doesn’t carry that volume every year. A series of dams store water for steady use during dry years, but periods of sustained drought can drop reservoir levels considerably. The Western United States has experienced consistent drought conditions since 2000.

Western states are using more water than the Colorado River can provide. As water levels drop, many places in the West are accumulating a water debt. That debt will continue to grow as population increases, causing changes to ecosystems and to the land itself. It is among the most serious issues facing Arizona today.

The fate of this river is closely tied to the future of our state. Tab through these pages to explore the facets of Colorado River.

Colorado River flow information from the U.S. Geological Survey and National Geographic.

Allocation: The Law of​ the River

A series of laws have governed water use in Arizona since miners first appeared in the 1800s. The first laws that allocated Arizona’s rivers and streams for use by prospectors and farmers, basically a “first come first serve” directive known as the doctrine of prior appropriation, is still largely the basis of water use in the West.

Arizona has to work nationally and internationally to manage our water supply. There are hundreds of water use laws on the books. The Water History Timeline offers a comprehensive list of laws and a history of water in Arizona.

The Law of the River refers to a series of laws specifically pertaining to management of the Colorado River.

The Colorado River Compact

The foundation of the current river law is the Colorado River Compact (CRC), an agreement between the seven river states first conceived in 1922. The CRC was developed to preserve the water rights of the upper basin states, where most water originated, and lower basin states, which had the most need for water.

The CRC splits allocation between the Upper and Lower basins in half, with each receiving 7.5 million acre-feet per year. The Boulder Canyon Project Act allocated 2.8 million acre-feet of river water to Arizona, 4.4 million to California, and .3 million to Nevada. Arizona was the last state to agree to this allotment, signing the CRC in 1944.

Unfortunately, the volumes of allocation were proposed when river flows had been consistently higher than average. These volumes are not sustainable today, and our water debt continues to grow. The Colorado River is overallocated.

The Colorado River Basin Project Act (Central Arizona Project)

Learn how CAP helps recharge aquifers and delivers water to central Arizona on the Central Arizona Project page.

Water can’t be used if it cannot get to where it’s needed. Historically, Arizona did not use its full share of the Colorado River, instead drawing water from the Gila River, a Colorado tributary. Two years after signing the CRC, Arizona started lobbying to build the Central Arizona Project, a canal system to move water from the Colorado to central and southern locations. However, California was drawing Arizona’s unused share of the Colorado. Under the doctrine of prior appropriation, California’s use of Arizona’s allocated water gave California de facto rights to it. However, The Arizona v. California U.S. Supreme Court Decision of 1964 decided that Arizona’s use of the Gila did not forfeit rights to our full 2.8 million acre-foot allocation. In 1968, The Colorado River Basin Project Act authorized numerous development projects of the Colorado River, including the Central Arizona Project.

Storage and Dams​

The Upper Colorado River Storage Project

In 1956, the Colorado River Storage Project authorized four dams on the Upper Colorado and its tributaries. These dams store up to 34 million acre-feet of water for use by the Upper Colorado Basin through a series of reservoirs created by the Glen Canyon Dam on the Utah/Arizona Border, Flaming Gorge in on the Green River in Utah, Navajo on the San Juan River in New Mexico, and the Wayne N. Aspinall Storage Unit on the Gunnison River in Colorado. During periods of low river flow, water from these reservoirs is released to Lees Ferry to meet its water obligation to the Lower Colorado Basin.

Glen Canyon Dam, located 15 miles upstream from Lees Ferry near Page, Arizona, stores more water than the other three reservoirs combined. The 710-foot-high dam flooded Glen Canyon to create Lake Powell, now the second largest artificial lake in the country.

Storage Dams on the Lower Colorado

Hoover Dam sits in the Black Canyon, found in the northwest corner of Arizona on the Nevada border. Once known as Boulder Dam, it was constructed during the Franklin Roosevelt administration between 1931–1936 to form Lake Mead, a reservoir with the capacity to store the entire average flow of the Colorado River for two years. The dam remains to this day a source of hydroelectric power for Arizona, Nevada, and Colorado, with a generation capacity of 4 billion kilowatt hours every year. However, generation has decreased as lake levels have dropped.

Davis Dam, forming Lake Mohave, was built to regulate water to Mexico as a result of the Mexican Treaty of 1944. The zoned earthfill structure has the capacity to generate up to 240 megawatts of power.

Parker Dam is commonly called “the deepest dam in the world.” Almost three-quarters of the dam’s 320-foot height is below the riverbed, leaving only about 85 feet of the concrete arch structure that forms Lake Havasu visible. It was constructed in 1938 to be a source for low cost hydroelectric power. About two miles upstream from the dam is the Metropolitan Water District`s W. P. Whitsett Intake Pumping Plant for the Colorado River Aqueduct that ends near Riverside, California. Lake Havasu is also the starting point for the Central Arizona Project, commencing at the Mark Wilmer Pumping Plant near Lake Havasu City.

Diversion Dams

A series of diversion dams along the Lower Colorado provide irrigation water to agricultural areas in Mexico, Arizona, and California. Two of particular importance are Morelos Dam, near the U.S-Mexico border, which diverts water to Mexico's Mexicali Valley; and the Imperial Dam and Desilting Works, which controls the flow of water into the All American and Gila Gravity Main Canals. Learn more about these important structures and their uses from the Bureau of Reclamation.

 Information from the U.S. Bureau of Reclamation.

Water to Mexico

An International River

Since 1944, Mexico has been entitled to 1.5 million acre-feet of the Colorado’s annual flow via the Mexican Water Treaty. However, in periods of low flow, water increases in salinity. By the 1960s, water that reached Mexico had a salt content of 1500 parts per million, rendering it too salty for drinking or even for agriculture. (Today, the river’s salinity is about 750 parts per million.)

Minute 242 of the U.S.-Mexico International Boundary and Water Commission of 1973 required the U.S. to reduce the salinity of water delivered to Mexico past the Morelos Dam. The Colorado River Basin Salinity Control Act of 1974 authorized construction of the Yuma Desalting Plant.

Yuma Desalting Plant

The plant is administered by the Yuma Area Office of the U.S. Bureau of Reclamation, the same office responsible for overseeing water distribution to the Imperial, Coachella, and Yuma Valleys, important agricultural areas known for winter vegetables.  Without the plant in operation, agricultural runoff and Colorado River excess is directed to the Wellton-Mohawk Bypass Strain, a bi-national canal that empties into the Cienega de Santa Clara, an important wetland in Mexico. When the plant is running, purified water can be restored to the Colorado and delivered to Mexico as part of the U.S. water obligation. Currently, the U.S. meets its water obligation by releasing water from the Lake Mead reservoir.

Yuma Desalting Plant is the largest desalinization plant in the country. Completed in 1992, its periods of operation have been sporadic but its potential has long been celebrated. Water officials in several states and Mexico believe desalinization is an important facet of long term water conservation.

In 2008, municipal utilities from California, Nevada, and Arizona approached the Bureau of Reclamation with a proposal to help operate the plant. The pilot run, where the plant was operated for 365 days over 18 months, was recently completed successfully. The test came in 31% under budget and conserved 30,496 acre-feet of water.

Ecosyst​ems and Conservation

See why invasive species such as the Quagga Mussel are dangerously upsetting ecosystems.

The Multi-Species Management Conservation Program plans to restore 81,000 acres of wetland habitat in the Lower Colorado River Basin.

The Colorado River supports wetland ecosystems essential to wildlife diversity in Arizona. An interdependent system of animals and plants, predators and prey, desert dwellers and migrating species, depend on the wetlands of the Colorado. Overuse of the river is causing the wetlands to disappear. In the 1970s it became apparent that species of birds, fish and even the desert tortoise were dying due to habitat loss. Meanwhile, non-native plants and animals, known as invasive species, were taking over. Today numerous projects are aimed at restoring the Colorado River habitat and controlling invasive species. A few are described below.

Multi-Species Management Conservation Program

In 1994 the U.S. Fish and Wildlife Service (USFWS) designated large parts of the Lower Colorado as critical habitat for four endangered and two threatened species. The states using the Lower Colorado River (California, Arizona, and Nevada) partnered with each other to develop a plan to protect the ecosystems around the Lower Colorado and to expand the populations of vulnerable species.

In 2006, USFWS and 56 partner agencies unrolled a cooperative effort called the Lower Colorado River Multi-Species Conservation Program (MSCP). This $626 million cooperatively funded plan takes a different approach than some conservation programs aimed at protecting one or two specific species. Instead of focusing on individual species, it focuses on rebuilding habitat to benefit entire ecosystems. By looking at the whole system instead of a few specific parts, the MSCP can restore balance to the ecosystem and slow the decline of other species dependent on the Lower Colorado.

The MSCP will run through 2055 to protect 11 square miles in Arizona, California, and Nevada and will construct 8,100 acres of habitat. Four types of riparian zone will be restored: aquatic systems, emergent marshes, lower terrace cottonwood and willow woodlands, and upper terrace mesquite thickets, or bosques.

Yuma East Wetlands Project

Once introduced to reclaim streambanks and prevent erosion, the tamarisk, also known as salt cedar, chokes native plants from riverbanks and is now considered a destructive invasive species.

The Yuma East Wetlands Project has removed non-native vegetation and developed pedestrian walkways between the confluence of the Gila and Colorado rivers and the Ocean to Ocean Bridge. Public uses for the 1,400 acre area include fishing, beaches, walking and hiking trails, and wildlife observation. Ongoing projects on the preserve include the creation of equestrian areas, walking paths, lakes, bird sanctuaries, hummingbird and butterfly gardens, tree farms, and revegetation. To date, the Yuma West Wetlands Project has received over $2 million in local, state, and federal grant funds.

Cocopah Restoration of the Limitrophe

The Limitrophe segment stretches 23 miles across the U.S.—Mexico border. Ten miles are on Bureau of Reclamation land while 12 miles wind through reservation land belonging to the Cocopah Indian Nation.

With funds from EPA, the Bureau of Indian Affairs and the Bureau of Land Management Reclamation, the Cocopah Indian Nation restored over 200 acres of riparian habitat in the Limitrophe by removing stands of invasive tamarisk and restoring native cottonwood, willow and mesquite. An additional 150 acres are under restoration with funds from the Department of Homeland Security, the USFWS, and the National Fish and Wildlife Foundation. This project works in conjunction with the Yuma East Wetlands Project.

Powell Expedition

Tau-gu, chief of the Paiutes, overlooking the river with John Wesley Powell. Circa 1873.

John Wesley Powell and an intrepid crew of nine were the first explorers of the entire length of the Colorado River. Powell was a scientist and naturalist who had explored many states in the Midwest during his long years of schooling, teaching, and putting himself through college. Despite the loss of his right forearm in the Civil War Battle of Shiloh, Powell undertook a remarkable journey through one of the last unexplored places in the American west.

Despite rumors of previous failed river runs, Powell believed the trip was possible if the right preparations were made. He spent time talking to American Indians who were familiar with the river and designed several small boats that he believed would meet the party’s needs. His crew consisted of eight mountain men who were experienced in living off the land and his brother, Walter. His exploration was funded by private sources and the Illinois State Natural History Society.

On May 24, 1869, Powell and his party departed from the Green River Station, Wyoming Territory. The party encountered many perils along the journey of over 1,000 miles. Some food supplies were lost and game was scarce. Scientific equipment was damaged. Four crew members deserted the expedition—one after just one month and three others just two days before the river trip concluded. On August 30, Powell and five men in two boats emerged at the mouth of the Virgin River in Arizona. They had made it through!

The journey had been rough, and Powell was not satisfied with the small amount of scientific information he had gathered. A number of collected specimens had been left behind in caches, and his damaged instruments hindered measurements. In 1871, Powell completed preparations for a second survey of the river. This expedition included a surveyor, Professor Almon H. Thompson, and an experienced photographer, E. O. Beaman. This voyage reaped both popular and scientific benefits. Professor Thompson was able to complete a topographic map of the Grand Canyon region, and the Smithsonian Institution published Powell’s account of the expedition in 1875. Many photographs taken by Beaman were made into stereograms and proudly displayed in living rooms across America. The last frontier of the continental American west was opened to public consciousness.

Information from  the National Park Service’s Cultural Resources Programs.


From Lees Ferry on the Colorado Plateau to the Morales Dam on the border to Mexico, the lakes, wildlife refuges, and stretches of waterway call adventurers and leisure seekers alike.

Lees Ferry

Lees Ferry

A natural corridor between Utah and Arizona, Lees Ferry is the only place within Glen Canyon where visitors can drive right up to the river, enjoy trout fishing upstream to Glen Canyon Dam or hiking or backpacking.

Grand Canyon

Grand Canyon

Whether you enter by foot, mule or boat, the Grand Canyon is a spectacular experience. But those who have rafted through the canyon—about 20,000 people a year—say there’s nothing like it anywhere. Learn more about Grand Canyon recreation.

Lake Mead

The largest man made reservoir in the United States, formed in the 1930s with the creation of the Hoover Dam.

Lake Mead is one of America’s most popular recreation areas. Formed by the Hoover Dam in Black Canyon, the lake boasts a year round season of water fun. Nine million visitors gather every year to enjoy swimming, boating, and fishing only 50 miles from Las Vegas. Many visitors stop to enjoy a tour of the inner workings of Hoover Dam, for many years an engineering marvel.  

Lake Mead National Recreation Area is the nation’s first national recreation area and includes Lake Mojave.

The Yuma Area

Located near the original townsite of Castle Dome Landing, Martinez Lake boasts two resorts.

Water fun abounds around Yuma. It’s a great way to kill the heat in summer temperatures that can soar past 110°F. The Yuma area has over a dozen wetlands, wildlife refuges, and lakes. Enjoy angling off the Morales Dam dredge launch or birdwatching at Mittry Lake Wildlife Area. Hunting and boating are also popular pastimes.